1. Field of the Invention
The present invention relates to an apparatus and method for recovering carbon dioxide from flue gas using ammonia water, and more particularly, to an apparatus and method for recovering carbon dioxide from flue gas using ammonia water, in which absorptive heat, generated in the course of recovering carbon dioxide from flue gas using ammonia water, is effectively dissipated, and the generation of ammonia salt is minimized, thereby increasing recovery efficiency of carbon dioxide.
2. Description of the Related Art
Generally, methods of absorbing and recovering carbon dioxide from flue gas include adsorption, absorption, and membrane separation.
Among these methods, the absorption method is a process of absorbing and recovering carbon dioxide from flue gas using an absorbent. As such, the absorbent is typically exemplified by an amine-based compound. However, the case where the amine-based compound is used as the absorbent suffers because the amine-based compound is expensive, undesirably increasing treatment costs, and also, because a mechanical system for absorbing and separating carbon dioxide from flue gas is corroded by the amine-based compound, undesirably requiring maintenance.
For these reasons, these days, as an absorbent for absorbing and recovering carbon dioxide from flue gas, ammonia water is receiving attention. This is because ammonia water is relatively inexpensive and mitigates the corrosion problems afflicting the mechanical system for absorbing and separating carbon dioxide from flue gas, which are not experienced when using the amine-based compound. Thus, when ammonia water is used as an absorbent, the total system scale is decreased, and energy consumption may be reduced.
Bai and Yeh reported a method of absorbing and recovering carbon dioxide using ammonia water (Ind. Eng. Chem. Res. 1997, vol. 36, pp. 2490), and also, Resnik et al reported a process for absorbing carbon dioxide using ammonia water, called an aqua ammonia process (Int. J. of Env. Technology & Management, vol. 4, No. 1).
Korean Patent No. 10-0703999 discloses a process for absorbing carbon dioxide, comprising supplying 5˜15 wt % of an ammonia-water solution to the upper portion of a first absorption column at 20˜40° C., supplying a carbon dioxide-containing gas mixture to the lower portion of the first absorption column so that carbon dioxide contained in the gas mixture is absorbed in ammonia water, transferring ammonia water, to which carbon dioxide is absorbed, to a stripping column so that carbon dioxide is stripped through heating to 70˜88° C., cooling the ammonia water from which carbon dioxide is stripped using a heat exchanger to thus circulate it again to the first absorption column, and supplying stripped carbon dioxide to a third absorption column so that ammonia contained in carbon dioxide is removed using water, thereby absorbing carbon dioxide.
However, the absorption process disclosed in the above patent is similar to a conventional process (Int. J. of Thermodynamics, vol. 7, pp. 173-181). As well, although problems related to the loss of ammonia are solved by providing each of the absorption column and the regeneration column with an additional washing column to recover ammonia therefrom, ammonia partially reacts with carbon dioxide to thus produce ammonia salt at low temperatures, thereby causing line clogging problems in the absorption column and the regeneration column.
Water, which is used to recover ammonia gas from the absorption column and the regeneration column, contains a large amount of ammonia. This ammonia is concentrated again in a concentration column, but ammonia thus concentrated is introduced into the lower portion of the regeneration column, and thus the concentration of ammonia is difficult to maintain uniform in the regeneration column. Further, water containing ammonia is fed into the upper portion of the regeneration column. Accordingly, the case where a reboiling process is performed in the regeneration column is problematic in that a lot of energy must be supplied.
Due to the generation of absorptive heat in the course of absorbing carbon dioxide using ammonia water, the temperature inside the absorption column is increased. When the temperature is increased, ammonia contained in the ammonia water is volatilized before carbon dioxide is collected in the absorbent, and is then discharged outside the absorption column along with flue gas, drastically decreasing carbon dioxide absorption efficiency in the absorption column.
Further, the above conventional methods are mainly related to the amount of carbon dioxide that is absorbed by ammonia and such reaction rates based on the fact that ammonia is used to absorb carbon dioxide, and may thus be experimentally performed, in which the ammonia water used has a very high ammonia concentration ranging from 5% to 21%.
However, when high-concentration ammonia water is applied to an actual process, problems attributable to the highly volatile nature of ammonia occur. That is, because ammonia is highly volatile, it is easily volatilized even when allowed to stand in the air. Hence, the case where high-concentration ammonia water is used is disadvantageous because a considerable amount of ammonia comes into contact with gas in the upper portion of the absorption column before it functions as an absorbent in the absorption column, and is thus volatilized.
Moreover, as mentioned above, the temperature inside the absorption column is increased due to the generation of absorptive heat in the course of absorbing carbon dioxide using ammonia water, and this increase in the temperature further promotes the volatilization of ammonia, remarkably deteriorating the effects of using high-concentration ammonia water.